Several freshwater invertebrate and vertebrate prey species rely on chemosensory cues, including noninjury released disturbance cues, to assess and avoid local predation threats. The prevailing hypothesis is that a pulse of ammonia released by disturbed or stressed prey functions as the disturbance cue. Here, we test this hypothesis in two phylogenetically distant freshwater prey fishes, convict cichlids and rainbow trout. In our first experiment, we measured NH 4 + concentration in tanks containing shoals of cichlid or trout before and after exposure to a realistic model predator (or left undisturbed). We failed to find an increase in ambient NH 4 + concentration for either cichlids or trout. In our second experiment, we exposed cichlids or trout to NH 4 + at 0.1 or 0.5 mg L -1 (or a distilled water control) and measured the change in antipredator behaviour (time moving, foraging rate and area use). We found no consistent increase in antipredator behaviour in response to NH 4 + . In our third study, we exposed cichlids and trout to the disturbance cues of cichlids or trout (versus the odour of undisturbed donors). We found significant increases in antipredator behaviour, regardless of donor species, for both cichlids and trout. Thus, the results of our first two experiments do not support the hypothesis that ammonium functions as a disturbance cue in prey fishes. However, the results of our final experiment do confirm the use of disturbance cues in convict cichlids and rainbow trout and support that hypothesis that the disturbance cue is indeed some generalized metabolic byproduct.

Inequality in male and female numbers may affect population dynamics and extinction probabilities and so has significant conservation implications. We previously demonstrated that Brown‐headed Cowbird Molothrus ater brood parasitism of Song Sparrows Melospiza melodia results in a 50% reduction in the proportion of female host offspring by day 6 post‐hatch and at fledging, which modelling demonstrated is as significant as nest predation in affecting demography. Many avian brood parasites possess special adaptations to parasitize specific hosts so this sex‐ratio effect could be specific to the interaction between these two species. Alternatively, perturbations associated with brood parasitism per se (e.g. the addition of an extra, larger, unrelated nestling), rather than a Cowbird nestling specifically, may be responsible. We experimentally eliminated the effects of Cowbird‐specific traits by parasitizing nests with a conspecific nestling rather than a Cowbird, while otherwise emulating the circumstances of Cowbird parasitism by adding an extra, larger (2‐day‐older), unrelated Song Sparrow nestling to Song Sparrow nests. Our parasitism treatment led to few host offspring deaths and no evidence of male‐biased sex ratios by day 6 post‐hatch. However, after day 6, female nestling mortality rates increased significantly in experimentally parasitized nests, resulting in a 60% reduction in the proportion of females fledging. Cowbird‐specific traits are thus not necessary to cause female‐biased host nestling mortality and far more general features associated with Cowbird parasitism instead appear responsible. Our results suggest, however, that Cowbird‐specific traits may help accelerate the pace of female host deaths. The conservation implications of our results could be wide reaching. Cowbirds are unrelated to all their hosts, are larger than the great majority, and a Cowbird nestling's presence can mean there is an extra mouth to feed. Thus, sex‐biased mortality in parasitized nests could be occurring across a range of host species.